Manufacturers of internal combustion engines are under constant pressure to achieve reductions in the quantities of harmful gases and particulates that are produced by the engine. At the same time, it is necessary to maintain current levels of engine performance to which consumers have become accustomed. In order to satisfy these requirements, it is essential both to monitor accurately and to control precisely the operational parameters of the engine.
The ability to measure accurately the mass of air flow that enters into the engine is critical to the efficient operation of modern engine management systems. Typically, an air mass flow (AMF) sensor is employed to perform this function.
Air mass flow sensors typically output a variable voltage that is proportional to the amount of air entering the engine cylinders via the air inlet of the engine. Some types of air mass flow sensors may also output a variable frequency voltage based upon the mass of air flowing therethrough. One approach taken to measure the mass of airflow is based on the so-called “hot wire” principle, in which a voltage is applied to an electrically conductive element in the form of a wire. The wire is disposed in an airflow sampling channel and is heated by an electrical current that is induced by the applied voltage. As air flows across the wire, the temperature of the wire is reduced and control circuitry will act to increase the current flowing through the wire in order to maintain a predetermined temperature. The air mass flow sensor outputs a voltage proportional to the current flow to an engine management system. The engine management system will derive a value of air mass flow from the received voltage value by reference to an air mass flow sensor characteristic.
A typical air mass flow sensor characteristic is shown in FIG. 1. The ordinate axis denotes the air mass flow in grams per second (g/s) and the abscissa axis denotes the corresponding voltage (V) that is output by the sensor. Line A represents the air mass flow sensor characteristic as calibrated and stored in the engine management system. It will be appreciated that the relationship between voltage and air mass flow is non-linear.
A disadvantage of air mass flow sensors is that their characteristic is susceptible to drift over time. As a result, the voltage output from the sensor, which is input to the engine management system, will be interpreted as an incorrect value of air mass flow. The problem is exacerbated in applications where the air mass flow sensor is required to measure accurately a wide range of flow rates, for example on large capacity engines. It can be seen from FIG. 1 that for a specific value ‘X’ of air mass flow, the air mass flow sensor will output gradually decreasing values of voltage, V1, V2, and V3 respectively, as the time from the last sensor calibration increases, as shown by curves A, B and C, respectively.
The error-prone voltage signal that is output from the air mass flow sensor adversely affects the ability of the engine management system to regulate accurately other engine operating parameters, such as the air-fuel ratio, that are vital for efficient combustion. Therefore, critical operational factors such as engine fuel economy, engine performance and exhaust emissions are likely to be influenced undesirably.
Although it is possible to re-calibrate the air mass flow sensor periodically, this would be inconvenient and time consuming since it would be necessary to present the vehicle to a re-calibration specialist to have the work carried out.